Speaker
Description
The electric dipole response of nuclei is dominated by the collective excitation mode known as the giant dipole resonance. This resonance accounts for most of the electric dipole strength and can be interpreted as a harmonic oscillation of protons against neutrons. However, a question arises when considering the addition of extra neutrons to nuclei: How does this impact the dipole response?
To address this question, extensive studies have been conducted to investigate the dipole response of exotic nuclei through virtual photon scattering experiments. Experimental evidence demonstrates the presence of significant dipole strength near the neutron threshold in neutron halo nuclei, where the last neutron is weakly bound. This strength is attributed to the non-resonant excitation of a weakly bound neutron into continuum. In medium-heavy neutron-rich nuclei characterized by relatively high neutron separation energy, a distinct dipole strength known as the pygmy dipole resonance (PDR) emerges at an excitation energy of 6 to 10 MeV. Despite the comprehensive investigations, the nature of the PDR remains uncertain.
In this contribution, we present an experimental methodology for probing the electric dipole response using virtual photon scattering, along with the responses observed in various neutron-rich nuclei. Furthermore, we discuss the evolution of the PDR across the Calcium isotope chain, incorporating our most recent experimental findings.
This research is supported in part by JSPS Grant-in-Aid for Scientific Research Grants No. JP21H01114.
